This invention relates generally to fiber optical communication systems, and, more particularly, to techniques for coupling light of different modes or modal groups into or out of a multimode optical fiber. An optical fiber is basically a cylinder made of transparent dielectric materials. A central region, referred to as the core, is surrounded by one or more annular cladding regions, which are usually surrounded by a protective jacket. In step-index fibers, the core has a fixed refractive index and the cladding has a fixed refractive index lower than that of the core. In simple terms, light rays launched into the fiber tend to be guided along it because of repeated total internal reflections at the cylindrical boundary between the core and cladding materials.
Electromagnetic light fields traveling in the fiber or being scattered by the fiber can be considered as superpositions of simpler field configurations, referred to as the modes of the fiber. The mode concept can be expressed in mathematical terms, but for purposes of explanation of the invention, a discussion in terms of geometric optics is more appropriate. Light rays entering a fiber at relatively low angles of incidence with respect to the longitudinal axis of the fiber are referred to as the lower-order modes of the fiber. Rays entering at higher angles of incidence are referred to as the higher-order modes.
Some rays entering the fiber at very high angles of incidence can be characterized as "leaky," meaning that there will not be total internal reflection of the rays within the fiber, and still others may be characterized as "unguided," meaning hat most of their energy will pass into the cladding and either be absorbed there or be radiated out from the fiber. The invention is concerned with guided lower-order modes and with guided higher-order modes. In communication systems, the different modal groups may be used as different information channels. This is a form of multiplexing, in which a group of lower-order modes is used to carry one information signal and a group of higher-order modes is used to carry another information signal.
In one type of bimodal communication system, the higher-order modes are used not to carry useful information, but rather as an intrusion detection device. When a bend of small radius is applied to an optical fiber, some of the light radiates from the core and may be used to decode the information signal being carried by the fiber. One technique for detecting such intrusion is to use the higher-order modes to carry a signal referred to as a masking signal or a monitor signal. If a fiber is bent to cause radiation of the transmitted light, the higher-order modes will radiate from the fiber core to a much greater degree than the lower-order modes. Monitoring the power ratio at the receiving end of the fiber allows the detection of intrusion of this type.
It essential in such a multiplexing arrangement that separate light signals be launched as lower-order and higher-order modal groups. Any cross-talk between the two modal groups will render them ineffective for multiplexing purposes. In the past, the only technique for launching bimodal signals into a single fiber was to launch a lower-order-mode signal from a light source disposed on the central axis of the fiber, and to launch a higher-order-mode signal from a light source displaced from the axis, to provide higher angles of incidence. Unfortunately, in such an arrangement it is difficult to control with any precision the amounts of light that are launched at various angles into the fiber. For example, some of the light from the centrally located source will be launched as higher-order modes in the fiber if the core-cladding interface of the fiber is not a perfect step index.
It will be appreciated from the foregoing that there is a significant need in fiber-optic communication systems for a technique that will reliably launch two separate light signals into a fiber as two different modal groups. The present invention fulfills this need.